A water-absorbing resin has hitherto been widely used in sanitary materials such as a paper diaper, a sanitary napkin etc., and industrial materials such as a water blocking material for a cable etc. A hydrolysate of a starch-acrylonitrile graft copolymer, a neutralized material of a starch-acrylic acid graft copolymer, a saponified material of a vinyl acetate-acrylic acid ester copolymer, a partially neutralized material of a polyacrylic acid and the like are known as the water-absorbing resin.
In recent years, in the sanitary materials such as a paper diaper, a sanitary napkin etc., the absorbent material tends to be thinned from a viewpoint of an amenity for use. Examples of reduction in a thickness of the absorbent material include a method for increasing a proportion of a water-absorbing resin in the absorbent material, a method for increasing the water retention capacity of the water-absorbing resin, and the like.
In the former method for increasing a proportion of the water-absorbing resin in an absorbent material, it becomes easy for water-absorbing resins to cause gel blocking to each other upon absorbing a body fluid. Therefore, the higher water-absorbing capacity under pressure of the water-absorbing resin is required in order to suppress the gel blocking between water-absorbing resins. However, in order to increase the water-absorbing capacity under pressure, it is generally necessary to increase a crosslinking density of the water-absorbing resin and, there is a problem that the water retention capacity of the water-absorbing resin decreases consequently.
In the latter method for increasing the water retention capacity of a water-absorbing resin, it is necessary to reduce a crosslinking density of the water-absorbing resin. Consequently, an uncrosslinked component is increased, and upon contacting with a liquid, a water-absorbing rate tends to decrease by making the lump state, and a gel strength decreases, and a slime matter (water-soluble substance) after absorbing the fluid, tends to be easily eluted. When the above water-absorbing resin is used as a diaper, this becomes cause for flow-back increase of a body fluid due to deficiency of a gel strength, a rash due to a water-soluble substance, and the amenity of a wearer is deteriorated.
As the technology for improving the above property in order to be favorably used in sanitary materials, the following technologies are known: a method for carrying out reverse phase suspension polymerization using a specific amount of a specific polymer protecting colloid and a specific surfactant (see Patent Literature 1), a method for performing reverse phase suspension polymerization in multi (two or more) steps (see Patent Literature 2), a method for performing reverse phase suspension polymerization in the presence of β-1,3-glucans to obtain a water-absorbing resin and then adding a crosslinking agent to the resultant water-absorbing resin to carry out a crosslinking reaction (see Patent Literature 3), a method for carrying out reverse phase suspension polymerization using a specific amount of a polymerization initiator persulfate (Patent Literature 4), a method for heating and mixing a water-absorbing resin precursor and a surface-crosslinking agent after the water-absorbing resin precursor is obtained by polymerizing it in an aqueous solution in the presence of phosphorous acid and/or a salt thereof (see Patent Literature 5), a method for mixing a water-absorbing resin with two or more kinds of crosslinking agents having different solubility parameters, and crosslinking the resin in a specific temperature range (see Patent Literature 6) and the like.
However, since these water-absorbing resins are such that, when the water retention capacity exceeds a certain value, a gel strength decreases, and a water-soluble substance increases, the performance of the absorbent material in which the above water-absorbing resins are used, is not sufficiently satisfactory.    Patent Literature 1: JP-A No. Hei 6-345819    Patent Literature 2: JP-A No. Hei 3-227301    Patent Literature 3: JP-A No. Hei 8-120013    Patent Literature 4: JP-A No. Hei 6-287233    Patent Literature 5: JP-A No. Hei 9-124710    Patent Literature 6: JP-A No. Hei 6-184320